The shortest ferry ride is only five minutes but that is only for Gulangyu Island residents until after 7 pm. Everyone else has to use the Dongdu international cruise ship terminal and this is a 20 minute trip. Apparently it can be very crowded and so may not have same-day tickets available. Having said that, when we were there in September we just went to the terminal around 10 am and were able to buy tickets for the next sailing. Note that you will need your passport to buy a ticket, unless you are a Chinese citizen.